Isolation of gamma benzene hexachloride



United States Patent Ofitice England, assignors to Imperial ChemicalIndustries Limited, a corporation of Great Britain No Drawing.Application August 10, 1954 Serial No. 449,009

Claims priority, application Great Britain August 14, 1953 7 Claims.(Cl. 260-648) This invention relates to improvements in the productionof organic compounds, particularly of the gamma isomer of benzenehexachloride, and more especially to a process for the isolation of thesaid gamma isomer from a mixture of the benzene hexachloride isomers.

Benzene hexachloride, as normally produced by simple additivechlorination of benzene in presence of light as catalyst, contains amixture of stereoisomers in which the gamma isomer is present to theextent of about 10% to 14% of the, total chlorination product, while thealpha isomer represents about 60% of this total product. In addition, itis now known that the proportion of gamma isomer formed in this processis influenced by varying the conditions of the reaction, and theproportion of gamma isomer in the crude material has been increased toover 20% by chlorinating at low temperatures ,in presence of certainchlorinated hydrocarbon solvents.

yIn most of the well known solvents the gamma isomer is from two to fourtimes as soluble as the alpha isomer and a number of processes forisolating the gamma isomer have been based on this fact. The procedurehas generally been to extract the crude isomeric mixture with a limitedamount of the selected solvent so that practically all the gammadissolves but only a comparativelyfsmall proportion of the alpha. A partof the gamma isomer can be obtained directly from this extract byboiling to remove part of the solventand then allowing to cool slowlywithout disturbance, when gamma isomer crystallizes from a mother liquorcontaining alpha isomer in supersaturation. Less than half the totalgamma content is recoverable in this way, however, and I in additionthis type of crystallization, although elfective, present obviousdisadvantages for large scale working.

We have now found that pure gamma isomer can be produced from the crudeadditive chlorination product of :benzene by a process which does notinvolve the retention of alpha isomer in a metastable state ofsupersaturation, and which is based on the discovery that the solubilityratio of gamma to alpha isomers in dimethyl carbonate is unusually low,being approximately 1:1, and. hence it is possible to recover gammaisomer by crystallization from this solvent of a gamma concentrate inwhich the ratio of gamma to alpha isomers is greater than unity. V V H vFor convenience it will be interpolated at this point that by the termsolubility ratio of gamma to alpha isomers in a particular solvent wemean the figure obtained by determining separately the solubility of thegamma isomer in grams per 100. grams of the solvent and similarly thatof the alpha isomer and then dividing the former figure by the latter. I

'Theprocess comprises treating a mixture of isomers of benzenehexachloride inswhich both the gamma and alpha isomers are present andin which the ratio of gamma isomer to alpha isomer is greater than unitywith dimethyl carbonate in at least sutficient amount and under suchconditions of temperature as to dissolve Patented June 2, 1 95 9 andretain in solution substantially all the alpha isomer together with anapproximately equal weight of gamma isomer while the remainder of thegamma isomer'which is undissolved or which, having been dissolved, isrecrystallized from the solution as a result of bringing the latter to asupersaturated state with respect to gamma isomer is separated off bydecantation, filtration, centrifuging or like means.

Obviously this process is equally adaptable to the production of a lesspure gamma fraction, for instance, a product containing say or of gamma.The only modification needed is to use less dimethyl carbonate at theextraction or crystallization stage so that some of the alpha isomereither remains undissolved with the gamma or, in the casewhere the wholeconcentrate is first dissolved, crystallizes along with the gamma.

According to the invention, therefore, a process for the separation ofthe gamma isomer of benzene hexachloride from a mixture of at least thealpha and gamma isomers in which the amount of the gamma isomer exceedsthat of the alpha isomer comprises treating the said mixture of isomerswith dimethyl carbonate under conditions of temperature andconcentration such that substantially all of the alpha isomer but only apart of the gamma isomer is taken into and finally retained in solutionin the dimethyl carbonate and thereafter separating the gamma isomerthat remains undissolved from the said solution.

A further feature of the invention is a modified process wherein thereis produced a less pure product, for instance, one containing 80 or moreof gamma isomet, by using appropriately less solvent at the extractionor crystallization stage.

The starting material for the process is thus a gamma concentrate inwhich the ratio of gamma isomer to alpha isomer is greater than unityand several procedures are known whereby such a concentrate may beproduced.

For instance, if in the normal process of additive chlorination ofbenzene the chlorination is continued long enough, crystals of the alphaand beta isomers separate out; if these are filteredofi and the filtrateevaporated to dryness, there is obtained a mixture of benzenehexachloride isomers containing considerably more gamma than alphaisomer. Again, if the chlorination is interrupted at an earlier stage,before any solid has separated, and the solution is concentrated byevaporation so as to cause on cooling precipitation of alpha and betaisomers, filtration, followed by removal of benzene from the filtrate byevaporation yields products wherein the proportion of gamma exceeds thatof the alpha isomer.

Products made by these methods, however, are liable to containundesirably large amounts of the delta isomer and impurities resultingfrom over-chlorination of some of the benzene. Materials that are freerfrom these undesired constituents may be obtained by carrying out thechlorination at low temperature in the presence of certain chlorinatedhydrocarbon solvents as described in US. patent application, Serial No.207,702, filed January 25, 1951, now abandoned. Preferably also theconcentration of free chlorine in the reaction system is carefullycontrolled as described in US. patent application, Serial No. 405,428,'filed January 21, 1954, now abandoned. Working up of these products inthe manner indicated above likewise yields concentrates that containmore gamma than alpha isomer.

Further, it is possible to take a crude mixture 'of benzene hexachlorideisomers and obtain therefrom a concentrate in which the gamma/alpharatio is greater than unity by treating the mixture with a limitedamount of a solvent in which the solubility of the gamma isomersignificantly'exceedsifor example, being between 2 and 4'times as greatas) that of the alpha isomer, so that all of the gamma isomer isdissolved whereas part of the alpha isomer remains undissolved or havingbeen dissolved crystallizes out again and so can be removed leaving asolution enriched in gamma isomer. This solution can be evaporated todryness leaving a solid concentrate suitable for treatment with dimethylcarbonate according to the process of the present invention.

One such method of obtaining a gamma concentrate suitable for use as astarting material in the process of the present invention is to extractthe crude benzene hexachloride, which may contain anything from to overof gamma isomer, with a limited amount of solvent such as methanol orethanol so that the gamma isomer is dissolved and a considerable amountof the less soluble alpha and beta isomers remains undissolved and canbe filtered off or otherwise removed, leaving a gamma-rich concentratein solution. Evaporation of the solvent will then give a product inwhich the proportion of gamma to alpha is greater than unity. Amongstother solvents which may be used instead of methanol or ethanol areother lower aliphatic alcohols, liquid homologues of benzene,chlorinated aliphatic hydrocarbons and esters of aliphatic carboxylicacids.

The concentrate obtained by any of these methods may contain between 30%and 60% of gamma isomer together with about one-third to half thisquantity of alpha isomer (delta isomer and other chlorinated productsare of course also present), and by further treatment with dimethylcarbonate the gamma isomer in excess of the amount approximately equalin weight to the alpha present can be isolated in practically purestate, without any necessity for crystallization from a solutioncontaining alpha in a metastable state of supersaturation. Thus it isadvantageous to start with a gamma concentrated in which the gamma/alpha ratio is as large as possible.

If the amount of gamma isomer crystallized from the dimethyl carbonatesolution corresponds to the full amount present in excess of the alphaisomer, it will be found that the product contains a certain proportionof alpha as impurity. To produce gamma isomer of a purity of 99% orover, therefore, it is necessary to take out rather less than thisactual excess and thus to use a little more than the amount of solventwhich, from the known solubilities of the separate individual isomers,would be expected to retain all the alpha isomer in solution. Since, asis described later, the gamma isomer remaining in solution is preferablyrecycled, it will be apparent that the total yield of gamma isomer Willnot be affected by the use of this additional solvent but that theamount of recycled material will become correspondingly greater.

Instead of using a gamma concentrate produced by one of the methodsdescribed above, it is also possible to use crystalline products richerin gamma isomer which have been obtained by fractional crystallizationfrom the solution produced by extracting the crude material with alimited amount of solvent. Such crystals may for instance containbetween 60% and 90% of gamma isomer, and the treatment with dimethylcarbonate otters a convenient method of purification of such materialfrom the contaminating alpha isomer.

The dimethyl cabonate treatment may be carried out in several ways, forexample, by complete dissolution of the concentrate and subsequentcrystallization of only the gamma isomer, or by extraction methodswhereby the alpha isomer and other impurities are dissolved togetherwith only a part of the gamma isomer, the rest of the gamma isomerremaining undissolved and thus being easily separable by methods such asdecantation, filtration or centrifuging.

For instance, the gamma concentrate may be completely dissolved in thesolvent at one temperature and the gamma isomer crystallized by coolingthe solution to a lower temperature, the quantity of solventused beingat least sufiicient to hold the alpha isomer in solution at thecrystallization temperature. The concentrate may be dissolved in thisamount of solvent by heating and the solution then cooled to bring aboutcrystallization or alternatively the gamma concentrate may be dissolvedin the solvent at room temperature and then the solution cooled topromote crystallization. Since however di methyl carbonate solidifies atjust over 0 C., the tem perature drop obtained by this latter procedureis insufficient to bring about the crystallization of much of the gammaisomer and it is preferable also to increase the degree ofsupersaturation by removing a certain amount of solvent. This can bedone by applying vacuum to bring about both the evaporation of solventand the cooling simultaneously. This last procedure is, of course,merely a specific case of the general method of dissolving theconcentrate wholly in the solvent then evaporating off such proportionof the solvent as it not required to keep the alpha isomer in solutionat the crystallization temperature.

It is also possible to carry out the process without completedissolution of the concentrate in the solvent, that is, by an extractionrather than a crystallization process. Thus an amount of solventsufficient only to dissolve the alpha isomer and part of the gammaisomer may'be stirred with the solid gamma concentrate and theundissolved gamma isomer may then. be separated by known methods such asdecantation, filtration or centrifuging.

It will be convenient to note at this point that not only is the ratioof the solubilities of the gamma and alpha isomers in dimethyl carbonatelow as compared with most other solvents, but so also is the gamma/betasolubility ratio, so that gamma isomer separated from alpha by theprocess of this invention is also substantially freed from beta isomer.

It will be apparent that the quantity of gamma isomer separated by thedimethyl carbonate treatment will be only a fraction of the total gammapresent, and that if the product is to be free from alpha isomer therewill necessarily remain dissolved in the mother liquor at least as muchgamma as alpha isomer. Further quantities of the gamma isomer left inthe mother liquor can be recovered therefrom by removing the solvent andtreating the product much in the Way that the original crude benzenehexachloride was treated. Thus it may be extracted with a solvent inwhich the gamma/ alpha solubility ratio is in the range 2:1 to 4:1,whereby more of the alpha isomer is eliminated, the residue then beingsubjected to another dimethyl carbonate treatment. Alternatively themother liquor may be evaporated further so that the solvent remaining isinsufficient to hold the greater part of the alpha isomer in solutionand thus a crop of crystals containing approximately equal amounts ofgamma and alpha isomers is obtained. These crystals are separated andrecycled to the first stage of the process comprising the concentrationof the crude material having a gamma/ alpha ratio of less than unity.The mother liquor from this second crystallization contains only a smallfraction of the total gamma isomer originally present but most of thedelta isomer and other impurities, and it is normally discarded asdelta-residue.

The preferred embodiments of the invention are combined processeswherein, starting from a crude chlorination product, technically puregamma benzene hexachloride of at least 99% purity is obtained with aminimum of extraction or crystallization procedures and a maximum degreeof recovery in a pure state of the gamma isomer initially formed.

In one such process the crude benzene hexachloride slurry obtained bychlorinating benzene in presence of methylene chloride, at a temperaturebelow 0 C., with a chlorine concentration in the range 0.1% to 0.3%weight/weight (compare United States application, Serial No. 405,428),which slurry contains benzene hexachloride isomers in proportions of 20%or more of gamma isomer, between 60% and of other isomers in addition tor nethylene chloride, unchanged benzene, and a small amount of dissolvedhydrogen chloride, is neutralized by stirring with soda ash, recyclematerial (referred to more fully hereinafter) is added and the mixtureis heated until all or nearly all the organic solids present have goneinto solution. A quantity of the solvent (methylene chloride andbenzene) is then evaporated and the solution cooled with stirring, whenthe greater part of the alpha and beta isomers separates out. These,together with any residual inorganic material, are filtered off, and thesolvent is completely removed from the mother liquor, leaving aconcentrate in which the gamma/alpha ratio is greater than unity. Thisconcentrate in fact contains about 60% of gamma isomer.

This concentrate is then dissolved in 1.6 times its own weight of hotdimethyl carbonate and the solution cooled With stirring to 20 C., whenpure gamma isomer crystallizes and is filtered ofi. Solvent isevaporated from the filtrate until the'solution contains 75% of benzenehexachloride and chlorinated impurities and the solution is 'cooledjto20 C., when a further crop of crystals is precipitated which containsabout 48% of gamma isomer (the remainder being mostly alpha). This isrecycled to the first stage of the process, that is, it is added to thecrude neutralized slurry befor'e evaporation of the solvent andseparation ofthe alpha and beta isomers. The mother liquor is evaporatedto recover the solvent and the mixture of benzene hexachloride isomersand impurities remaining is rejected.

The process can be Worked in batches but is particularly suitable forcontinuous Working. ,7

Another convenient way of carrying out the process, suitable for use asa continuous manufacture, is described in the following example whichillustrates but does not limit the invention.

Example Benzene and methylene chloride in molar ratio 3:4 are passedwith eflicient stirring through a reactor held at a temperature of 25C., and illuminated with light from a mercury arc lamp the mainconstituent of which is of Wavelength 4358 A., into which reactorchlorine is also passed at such a rate that the concentration of freechlorine in the solution is maintained sensibly constant at a value inthe range 0.1% to 0.3% by weight of the total Weight of the reactionmixture. The rate at which the benzene-methylene chlorine mixture passesthrough the reactor is adjusted so that 38% of the benzen ischlorinated. There is thus obtained a slurry consisting of the alpha andbeta isomers in a liquid consisting of alpha, beta, gamma and deltaisomers, some chlorinated impurities, unchanged benzene, methylenechloride and some dissolved hydrogen chloride. This slurry isneutralized by stirring with soda ash and at this point there is added amixture of benzene hexachloride isomers and chlorinated impurities morefully described below and referred to as recycle material. The wholemass is well mixed and filtered, the solids, consisting of alpha andbeta isomers and inorganic material, are rejected and the mother liquoris evaporated to yield a gamma concentrate suitable for treatment withdimethyl carbonate.

This gamma concentrate while still molten is dissolved in 0.7 times itsown weight of dimethyl carbonate at 50 C., and the solution is thencooled to 20 C., when gamma isomer of a purity exceeding 99%crystallizes out and is filtered oif, washed with dimethyl carbonate anddried.

The mother liquor is evaporated and the solvent recovered. The benzenehexachloride residue is divided into a major portion which constitutesthe recycle material referred to above and a minor portion that isrejected, the proportion of the latter being adjusted so that it carriesaway per unit of time as a purge an amount of the delta isomer andimpurities equal to that formed in that time in the chlorinationreaction.

In what has been described above the solvent has been consistentlyreferred to simply as dimethyl carbonate. It is to be understood,however, that it is not essential, in order to secure the benefits ofthis invention, to use pure or substantially pure dimethyl carbonate. Itis indeed possible to tolerate the presence of quite appreciableproportions of impurities or deliberate additions of other solventswithout wholly losing the benefit of the low gamma/ alpha solubilityratio upon which the success of the invention is based. Thus, forinstance, admixture with dimethyl carbonate of up to 40% of benzene,methylene chloride or methanol increases the gamma/alpha solubilityratio, but not beyond 2:1, so that even with such deliberately debasedsolvent the advantages associated with the use of dimethyl carbonate arestill largely obtained.

Our invention, therefore, includes also processes wherein instead ofdimethyl carbonate there is used a mixture of dimethyl carbonate withone or more of What we term, for convenience, the normal solvents forbenzene hexachloride, that is, solvents for which the gamma/ alphasolubility ratio has the more usual value of 2:1 to 4:1, usually 3:1 to4:1. Examples of such solvents are benzene and methylene chloride (smallproportions of which may be easily carried forward into the dimethylcarbonate from earlier stages in the process, namely from the productionof the gamma-rich concentrate), methanol, isopropanol and other solventsthe use of which in producing gamma-rich concentrates is described inBritish specifications Nos. 573,693, 586,439 and 586,442, and alsomixtures with water of suchof these solvents as are Water-miscible orform azeotropes with water or with water and dimethyl carbonate. Quitesubstantial proportions of such solvents can be tolerated with naturallysome loss of efliciency but without losing entirely the favorablegamma/alpha solubility ratio that is characteristic of dimethylcarbonate. Indeed, the invention contemplates the use of dimethylcarbonate pure or substantially pure or of any solvent mixturecontaining dimethyl carbonate provided the proportion of dimethylcarbonate in the mixture is such that the gamma/alpha solubility ratiofor that mixture does not exceed 2: 1.

What we claim is:

1. A continuous process for the isolation of the gamma isomer of benzenehexachloride which comprises photochlorinating benzene in presence ofmethylene chloride at a temperature below 0 C., with a chlorineconcentration in the range 0.1% to 0.3% by weight of the total weight ofthe reaction mixture until there is formed a slurry containing alpha andbeta benzene hexachlorides but substantially no gamma benzenehexachloride, neutralising the acid in the product, adding the mixtureof gamma and alpha isomers referred to hereinafter as the recyclematerial, heating until all or nearly all the organic solids presenthave gone into solution and a portion of the solvent has been evaporatedoif, cooling the solution to produce a precipitate containing alpha andbeta isomers but substantially no gamma isomer, filtering off thisprecipitate together with inorganic material arising from theneutralisation step, evaporating the filtrate to remove unchangedbenzene and solvent, dissolving the residue in hot dimethyl carbonate,cooling the solution with stirring and separating off the pure gammaisomer which crystallises and which constitutes the product, evaporatingpart of the solvent from the residual solution, cooling this solutionand returning the crystals that separate out (composed of gamma andalpha isomers and conveniently termed the recycle material) to the cycleby adding them to the neutralised slurry prior to the first evaporationof solvent.

2. A continuous process for the isolation of the gamma isomer of benzenehexachloride which comprises photochlorinating benzene in the presenceof methylene chloride at a temperature between 0 C. and minus 25 C.,with a concentration of free chlorine in the reaction medium in therange of about 0.1% to 0.3% by weight of the total weight of thereaction mixture until approximately 38% of the benzene is chlorinatedand a precipitate containing the alpha and beta isomers butsubstantially no gamma isomer is formed, neutralizing the acid in themixture, adding the residue referred to hereinafter as the recyclematerial, filtering off and rejecting the solid material, evaporatingthe mother liquor to remove unchanged benzene and solvent, dissolvingthe residual concentrate in dimethyl carbonate at elevated temperature,cooling to cause crystallization of gamma isomer of a purity of at least99%, filtering off these crystals which constitute the product,evaporating the mother liquor and returning a portion of the residue(termed the recycle material) to the cycle by adding it to theneutralized slurry immediately following neutralization, the proportionof the residue not so returned being adjusted so that it carries awayper unit of time as a purge an amount of the delta isomer and impuritiesequal to that formed in that time in the chlorination reaction.

3. A process for the fractional extraction of the gamma isomer- 0fbenzene hexachloride from a mixture containing the alpha isomer ofbenzene hexachloride and the gamma isomer of benzene hexachloridewherein the amount of said gamma isomer in said mixture is greater thanthe amount of said alpha isomer, which comprises in combination thesteps of (1) treating said mixture at a temperature of at least aboutroom temperature with an amount of dimethyl carbonate suflicient todissolve and retain in solution said alpha isomer, and an amount of saidgamma isomer approximately equal to the amount of said dissolved alphaisomer, but not all of said gamma isomer, and (2) after all of said,alpha isomer is dissolved,separating the purified undissolved gammaisomer from the mother liquor.

4. A process for the fractional extraction of the gamma isomer ofbenzene hexachloride from a mixture containing the alpha isomer ofbenzene hexachloride and the gamma isomer of benzene hexachloridewherein the amount of said gamma isomer in said mixture is greater thanthe amount of said alpha isomer, which comprises in combination thesteps of (1) dissolving said mixture in an amount of dimethyl carbonatesufficient to completely dissolve all of said mixture and form aninitial solution at a temperature of at least about room temperature upto the boiling point of the dimethyl carbonate, 2) super-saturating thisinitial solution with respect to the said gamma isomer, (3)crystallizing said gamma isomer from the super-saturated solution, alphaisomer remaining in the mother liquor solution, and (4) thereafterseparating said crystallized purified gamma isomer from the motherliquor.

5. The process of claim 4 wherein said initial solution issupersaturated by cooling.

6. The process of claim 4 wherein said initial solution issupersaturated by removing a portion of said dimethyl carbonatetherefrom.

7. The process of claim 6 wherein after said removal of said dimethylcarbonate, the residual solution is cooled.

References Cited in the file of this patent Slade: Chemistry andIndustry, Oct. 13, 1945, pages 314-9.

1. A CONTINUOUS PROCESS FOR TH ISOLATION OF THE GAMMA ISOMER OF BENZENEHEXACHLORIDE WHICH COMPRISES PHOTOCHLORINATING BENZENE IN PRESENCE OFMETHYLENE CHLORIDE AT A TEMPERATURE BELOW 0*C., WITH A CHLORINECONCENTRATION IN THE RANGE 0.1% TO 0.3% BY WEIGHT OF THE TOTAL WEIGHT OFTHE REACTION MIXTURE UNTIL THERE IS FORMED A SLURRY CONTAINING ALPHA ANDBETA BENZENE HEXACHLORIDES BUT SUBSTANTIALLY NO GAMMA BENZENEHEXACHLORIDE, NUE TRALIZING THE ACID IN THE PRODUCT, ADDING THE MIXTUREOF GAMMA AN ALPHA ISOMERS REFERRED TO HEREINAFTER AS THE RECYCLEMATERIAL, HEATING UNTIL ALL OR NEARLY ALL THE ORGANIC SOLIDS PRESENTHAVE GONE INTO SOLUTION AND A PORTION OF THE SOLVENT HAS BEEN EVAPORATEDOFF, COOLING THE SOLUTION TO PRODUCE A PRECIPITATE CONTAINING ALPHA ANDBETA ISOMERS BUT SUBSTANTIALLY NO GAMMA ISOMER, FILTERING OFF THISPRECIPITATE TOGETHER WITH INORGANIC MATERIAL ARISING FRM TEHNEUTRALISATION STEP, EVAPORATING THE FILTRATE TO REMOVE UNCHANGEDBENZENE AND SOLVENT, DISSOLVING THE RESIDUE IN HOT DIMETHYL CARBONATE,COOLING THE SOLUTION WITH STIRRING AN DSEPARATING OFF THE PURE GAMMAISOMER WHICH CRYSTALLISES AND WHICH CONSTITUTES THE PRODUCT, EVAPORATINGPART OF THE SOLVENT FROM THE RESIDUAL SOLUTION, COOLING THIS SOLUTIONAND RETURNING THE CRYSTALS THAT SEPARATE OUR (COMPOSED OF GAMMA ANDALPHAS ISOMERS AND CONVENIENTLY TERMED THE RECYCLE MATERIAL) TO THECYCLE BY ADDING THEM TO THE NEUTRALISED SLURRY PRIOR TO THE FIRSTEVAPORATION OF SOLVENT.